This invention relates to a circuit board comprising a high-strength ceramic-and-metal composite, as well as a process for its manufacture. More specifically, the invention relates to a power module circuit board having such a sufficiently high resistance to heat cycles that it is suitable for mounting integrated circuits and semiconductor components. The invention also relates to a process for the manufacture of such power module circuit board.
Ceramic circuit boards having conductor patterns have been extensively used as substrates for mounting power devices such as power transistors, IGBT (Insulated Gate Bipolar Transistor), IPM (Integrated Power Module) and power modules that generate large amounts of heat. In recent years, particular attention has been drawn to the application of AlN ceramic circuit boards having high heat conductivity and in order to fabricate quality AlN ceramic circuit boards, various design improvements have been added to such aspects as the manufacture of ceramic substrates and the formation of conductor patterns.
To take the manufacture of AlN ceramic substrates as an example, rare earth oxides typified by yttria (Y.sub.2 O.sub.3) or alkaline earth oxides typified by calcia are added as sintering additives in order to produce dense substrates; stated more specifically, AlN ceramic substrates having high heat conductivity that are currently sold on the market have Y.sub.2 O.sub.3 added in amounts of about 2-8 wt %.
In mass production of AlN ceramic substrates, from several to several tens of substrates are stacked and sintered at a time, with release agents such as boron nitride (BN) being commonly employed in order to prevent one substrate from adhering to another.
Conductor patterns are conventionally formed on the surface of ceramic substrates by the following three common methods: metallization in which a conductive paste is printed on the substrate surface and fired at an elevated temperature to form the intended conductor pattern (Unexamined Published Japanese Patent Application No. 149485/1990); direct bonding in which an AlN ceramic substrate is preliminarily treated at a temperature of about 1,000.degree. C. in air to form alumina on the substrate surface, followed by either heating in an inert gas atmosphere using an oxygen-containing copper plate or heating in an oxidizing atmosphere using an oxygen-free copper plate, to thereby create a eutectic solution of Cu.sub.2 O and Cu at the interface such that the AlN ceramic circuit substrate having alumina formed on the surface is joined to the copper plate so as to form the desired conductor pattern (Unexamined Published Japanese Patent Application No. 93687/1991); and brazing in which a brazing material is coated in a circuit pattern on the substrate surface and a copper part placed on the brazing material is heated so that the copper part is joined to the substrate, thereby forming the intended conductor pattern. Direct bonding and brazing are the two methods that have principally been employed to form conductor patterns in the manufacture of power circuit boards.
According to Unexamined Published Japanese Patent Application No. 258686/1990, AlN ceramic circuit substrates have a surface layer rich in the bleeding grain boundary phase constituents and impurities such as a release agent remaining on the surface will interfere with the effective joining between aluminum nitride and the formed conductor patterns; therefore, in order to form conductor patterns by metallization, the grain boundary phase constituents have to be removed by a suitable technique such as lapping and honing.
According to Unexamined Published Japanese Patent Application No. 149485/1990, the grain boundary phase constituents crystallizing out on the surface of an AlN ceramic substrate roughen the substrate surface to thereby lower the strength of the joint between the ceramic substrate and the conductor, as well as impairing the heat conductivity of the circuit board; to avoid these difficulties, the stated patent proposes that the grain boundary phase constituents be removed from the substrate surface by means of liquid solvents.
According to Unexamined Published Japanese Patent Application No. 93687/1991, the grain boundary phase constituents crystallizing out on the surface of an AlN ceramic circuit substrate are practically insensitive to an oxidation treatment and will remain on the substrate surface, thereby lowering the strength of adhesion between a surface coating layer (i.e., an aluminum oxide layer in the direct bonding method) and the ceramic substrate; to overcome this difficulty, the stated patent proposes that the surface coating layer be formed after removing the grain boundary phase constituents by applying a chemical surface treatment with an acidic solution.
As it turned out, however, the peel strength and the number of heat cycles that could be withstood by the ceramic circuit boards manufactured by the joining methods described above were not high enough to meet the requirements for use with recent versions of power modules; the peel strength was only about 10 kg/cm and the number of heat cycles was less than 100, both in terms of practically effective values.
In order to solve these problems, the assignee conducted intensive studies and found that the flexural strength of a ceramic substrate, the weight ratio between the oxygen to yttrium contents in the substrate, the residual boron nitride content on the substrate surface and the joining temperature had significant effects on the strength of joint; based on this finding, the assignee previously proposed a high-strength aluminum nitride circuit board having a peel strength of at least 30 kg/mm.sup.2 in Unexamined Published Japanese Patent Application No. 53624/1994.
Power module ceramic circuit boards have not only semiconductors and integrated circuits, but also macroscopic electrodes connected thereto and, hence, are subject to substantial forces and this has made it necessary to develop circuit boards having higher peel strength and being capable of withstanding more heat cycles.
The assignee disclosed in Unexamined Published Japanese Patent Application No. 53624/1994, supra, that when an aluminum nitride substrate in which the ratio between the X-ray diffraction intensity of boron nitride (I.sub.BN) present on the surface layer and the X-ray diffraction intensity of aluminum nitride (I.sub.AlN) was no more than 6.times.10.sup.-2 was joined to a copper plate with an active metallic brazing material interposed therebetween had a peel strength of at least 30 kg/mm.sup.2.
Recent models of power module ceramic circuit boards require that peel strengths of at least 30 kg/mm.sup.2 be exhibited in combination with the ability to withstand more than 30 heat cycles which is a typical value for the existing modules. However, the high-strength AlN circuit board previously proposed by the assignee cannot be manufactured as a copper-bonded circuit board that satisfies both requirements.